1. Field of the Invention
The invention relates to the preparation of chemically resistant and flame retardant foams from aromatic acid dianhydrides and aromatic polyisocyanates.
2. The Prior Art
Polymeric materials containing varying concentrations of imide linkages have been prepared heretofore in several manners. Frey, in U.S. Pat. No. 3,300,420 first proposed the preparation of this type of structure by causing aromatic polymeric isocyanates to react with dianhydrides at elevated temperatures while under agitation. A large selection of materials is suggested for optional use as modifiers, catalysts or accelerators. Included are primary, secondary, and tertiary amines, fatty acid amides, as well as alkali and heavy metal salts of weak organic acids, with lithium ricinoleate and sodium oleate being singled out as preferred materials for this purpose. In any event, Frey does not ascribe any particular significance to the choice of catalyst. One of the difficulties with the original process of Frey is that at the high temperatures involved, isocyanates form some cyclic trimers which eventually decompose to form carbodiimides. This competing reaction often leads to dark, brittle, and unpredictable products.
Attempts to improve the formation of polymeric foams containing imide linkages have involved typically the use of lower temperatures, specific catalysts and solvents as disclosed by Farissey et al., Miller, and more recently, by Rosser. The Farissey et al. approach, as disclosed in U.S. Pat. No. 3,562,189, is to use a dipolar aprotic solvent which permits the polymerization to proceed at room temperature. A large number of catalysts is suggested, such as are conventionally employed to facilitate the reaction of an isocyanate with an active hydrogen-containing compound. Of these, which include all kinds of inorganic and organic salts, only tertiary amines are specifically claimed. Similarly, Miller, in U.S. Pat. No. 3,489,696, teaches the formation of imide linkage-containing polymers from isocyanates and polycarboxylic acids by a process which calls for the absence of anhydrides. If the latter are used as starting materials, some water must be present to hydrolyze them. This imide-containing polymer synthesis process utilizes the heat of reaction of the polyurethane formation which has previously taken place. The polyurethane material is formed from isocyanates and active hydrogen-containing compounds in the presence of any one of a large number of catalysts. These catalysts include stannous chloride, stannous salts of carboxylic acids having 1 to 18 carbon atoms, many amines, and the like. Again, the materials produced, as claimed, contain a large proportion of non-imide linkages, such as urethane, urea, and the like, which ultimately diminish their heat and fire resistance.
The last pertinent art that needs to be discussed here is U.S. Pat. No. 3,772,216. The process disclosed therein by Rosser is that of forming polyimide foams from organic polyisocyanates and aromatic polycarboxylic acid derivatives such as dianhydrides, in the presence of water, an alkanolamine catalyst, and more than 2% by weight, preferably 10% by weight, of a siloxane-glycol copolymer. As can be imagined, the water and the amine, both individually and together, contribute to the occurence of various side reactions such as trimerization, urea linkage formation, and appearance of polyamines, the latter reacting with the dianhydride to the detriment of complete imidization. All the products of these secondary reactions are evident in the resulting foam, which rather than being a true polyimide, is a combination of polyurea, polyisocyanurate and polyimide not possessing, as shall be demonstrated, the thermal stability expected of true polyimides.
An object of this invention, therefore, is to provide a discriminating catalytic system which will categorically favor imide linkage when an aromatic tetracarboxylic dianhydride is allowed to react with an aromatic polyisocyanate.
Another object is to provide organic foams having a regular structure, good mechanical properties and unique thermal characteristics.
These and other objects which will become apparent later have been accomplished by the process that shall now be described.